A multi-carrier communication system, such as Discrete Multi-Tone (DMT) system, carries information from a transmitter to a receiver over a number of tones. Each tone may be a group of one or more frequencies defined by a center frequency and a set bandwidth. The tones are also commonly referred to as sub-carriers or sub-channels. As used herein, the terms tone and frequency may be used interchangeably.
DMT communication systems use a modulation method in which the available bandwidth of a communication loop, such as twisted-pair copper media, is divided into these numerous sub-channels. A communication loop may also be known as a communication channel. However, to avoid confusion, the term channel is used herein in reference to tones and frequencies, rather than physical communication media. The term communication loop is understood to refer generally to physical communication media, including copper, optical fiber, and so forth, as well as communication signal paths, including radio frequency (RF) and other physical or non-physical communication signal paths.
There are various sources of interference and noise in a multi-carrier communication system. Interference and noise may corrupt the data-bearing signal on each tone as the signal travels through the communication loop and is decoded at the receiver. The transmitted data-bearing signal may be decoded erroneously by the receiver because of this signal corruption.
In order to account for potential interference on the transmission line and to guarantee a reliable communication between the transmitter and receiver, each tone can merely carry a limited number of data bits per unit time. This number is related to a bit error rate (BER) for a given tone. The number of data bits or the amount of information that a tone carries may vary from tone to tone and depends on the relative power of the data-bearing signal compared to the power of the corrupting signal on that particular tone. The number of bits that a specific tone may carry decreases as the relative strength of the corrupting signal increases.
A reliable communication system is usually defined as a system in which the probability of an erroneously detected data bit by the receiver is always less than a target value. It is a common practice to model the aggregate sources of corruption associated with each tone as a single additive noise source with Gaussian distribution. Some exemplary sources of destructive noise sources include thermal energy, AM radio waves, and inter-loop cross-talk. Under the assumption of a Gaussian distribution, the signal-to-noise power ratio (SNR) becomes a factor in determining a bit rate—the maximum number of data bits a tone can carry reliably. Given a fixed bit rate (or data transfer rate) on a tone, the effects of noise sources on the communication channel should be reduced in order to reduce the error rate on that tone.
The SNR of a tone may decrease as the length of the communication loop increases. Hence, a short communication loop may likely have a higher SNR than a long communication loop, where the length of the communication loop refers to the distance of the communication signal path between the transmitter and the location at which the SNR is calculated or measured. Where a long communication loop has a low SNR and a nearby short communication loop has sufficient transmission power, the signal on the short communication loop may cause noise that interferes with the data-bearing signal on the long communication loop. This noise is known as cross-talk. In many cases, decreasing the transmission power of the signal on the short communication loop will not significantly eliminate the noise that results on the long communication loop.
One technology, Dynamic Spectral Management (DSM), is a fairly complex tool that may be used to jointly optimize rates for the entire binder. However, DSM can be complex and have high associated costs.
A conventional bit-loading algorithm usually loads the data bits on tones with the highest SNR. This is to achieve the maximum noise margin and/or minimum transmission power. Because the typical SNR profile in an ADSL line drops with frequency, a conventional bit-loading algorithm activates the low frequency tones with the highest SNR and, correspondingly, deactivates tones with the lowest SNR. This implementation can be referred to as an SNR-pruning method.
However, SNR-pruning schemes employed on short communication loops tend to concentrate transmission signals for the short communication loop on tones that overlap with the tones used for long communication loops. Although overall transmission power may be reduced with SNR-pruning, noise and interference on the long communication channel are not necessarily reduced.